In electrophotographic fuser systems, fuser roller overcoats are made with layers of polydimethylsiloxane (PDMS) elastomers, fluorocarbon resins and fluorocarbon elastomers. PDMS elastomers have low surface energy and relatively low mechanical strength, but is adequately flexible and elastic and can produce high quality fused images. After a period of use, however, the self-release property of the roller degrades and offset begins to occur. Application of a PDMS oil during use enhances the release property of the fuser roller surface but shortens roller life due to oil swelling. Fluorocarbon resins like polytetrafluoroethylene (PTFE) have good release property but less flexibility and elasticity than PDMS elastomers. Fluorocarbon elastomers, such as Viton.TM. and Fluorel.TM., are tough, flexible, resistant to high temperatures, durable and do not swell, but they have relatively high surface energy and poor thermnal conductivity.
Particulate inorganic fillers have been added to fluorocarbon elastomers and silicone elastomers to increase mechanical strength and thermal conductivity. High thermal conductivity is an advantage because heat needs to be efficiently and quickly transmitted from an internally heated core to the outer surface of the fuser roller to fuse the toners and yield the desired toner images. However, incorporation of inorganic fillers to improve thermal conductivity has a major drawback: it increases the surface energy of fuser roller surface and also increases the interaction of the filler with the toner and receiver. After a period of use, the toner release properties of the roller degrade and toner offset begins to occur due to roller wear and weak interaction between the filler and the polymer matrix. It would be desirable to provide a fuser member having a fluorocarbon elastomer overcoat layer containing thermally conductive inorganic fillers, but which still has a moderately low surface energy and good toner release property. In addition, it should be compatible with the functionalized polymeric release agent employed during fixing process.
Fuser members of fluorocarbon elastomer containing inorganic filler are disclosed, for example, U.S. Pat. No. 5,464,698 to Chen et al. which describes fuser rollers having a surface layer comprising fluorocarbon elastomer and tin oxide fillers. The fillers provide active sites for reacting the mercapto-functional polydimethylsiloxane. However, the inorganic fillers are not combined with a coupling agent and remain highly reactive with the toner and charge control agent, and this is undesirable.
U.S. Pat. No. 5,595,823 to Chen et al. describes fuser rollers having a surface layer comprising fluorocarbon elastomer and aluminum oxide fillers which also are not combined with a coupling agent and so are prone to high reactivity with toner and charge control agent which, again, is undesirable.
U.S. Pat. No. 5,017,432 to Eddy et al. describes a fluorocarbon elastomer fuser member which contains cupric oxide to interact with the polymeric release agent and provide an interfacial barrier layer.
Fuser members of condensation-crosslinked PDMS elastomers filled with metal oxides are disclosed, for example, in U.S. Pat. No. 5,401,570 to Heeks et al. This patent describes a silicone rubber fuser member containing aluminum oxide fillers which react with a silicone hydride release oil.
U.S. Pat. No. 5,480,724 to Fitzgerald et al. discloses tin oxide fillers which decrease fatigue and creep (or compression) of the PDMS rubber during continuous high temperature and high stress (i.e. pressure) conditions.
Some metal oxide filled condensation-ured PDMS elastomers are also disclosed in U.S. Pat. No. 5,269,740 (cupric oxide filler), U.S. Pat. No. 5,292,606 (zinc oxide filler), U.S. Pat. No. 5,292,562 (chromium oxide filler), and U.S. Pat. No. 5,336,596 (nickel oxide filler). All provide good results.
Unfortunately, as fuser rollers wear, the metal oxide fillers that are exposed react not only with the functionalized polymeric release agent, but also with the toner, paper substrate and charge control agent. Such reactions build up debris on the surface of the fuser roller, causing deterioration of toner release and great reduction in the life of the fuser roller. Thus, there remains a need for fuser members whose metal oxide fillers are made to enhance the interaction between elastomer and filler and also between the polymeric release agent and filler.